How does the milkweed beetle safely feed on a toxic plant?

Researchers have recently unlocked new insights into how the red milkweed beetle thrives on toxic plants, shedding light on the ecological, evolutionary, and potential economic impacts of insect-plant interactions. 

While the relationship between the red milkweed beetle and milkweed plants has been studied for nearly 150 years, scientists have now taken a major step forward by sequencing the beetle’s genome and examining genes tied to plant-feeding and other biological traits.

The research was conducted by Rich Adams from the Arkansas Agricultural Experiment Station, in collaboration with colleagues from the University of Memphis and the University of Wisconsin Oshkosh.

Red milkweed beetle genome

The researchers set out to sequence and assemble the entire genome of the red milkweed beetle (Tetraopes tetrophthalmus), a host-specialist. 

The research allowed them to compare the genome of this beetle with that of the Asian long-horned beetle (Anoplophora glabripennis), a host-generalist and invasive species that feeds on a variety of important trees.

“From a biological standpoint, there is a lot of correspondence that suggests that longstanding interactions between milkweed beetles and their toxic milkweed hosts should influence the biology of both interacting partners,” explained Adams. 

“But, to date, no one has assembled a milkweed beetle genome, which opens the door for targeting a lot of interesting questions at the interface between insect and plant.”

Co-evolution of plants and insects

Milkweeds and milkweed beetles have long been valuable models for studying ecology, evolution, developmental biology, and the biochemistry of toxins. They have also provided compelling evidence of co-evolution between insect and plant species. 

The timing of their evolutionary histories aligns, meaning that milkweed plants and their beetle counterparts evolved in tandem.

In their research, the team found that the red milkweed beetle has an expansion of genes from the ABC transport family, which may allow the beetles to feed on the toxic milkweed and sequester the plant’s toxins within their tissues. 

Milkweed plants are well-known for producing toxic latex cocktails that affect the balance of sodium, calcium, and potassium necessary for heart function. 

Safe interactions with a toxic plant

Adams emphasized that the genome analysis offers important insights into how the beetle has evolved to safely interact with its toxic host plants.

“Milkweeds produce a particularly nasty type of toxin called cardiac glycosides alongside other types of toxins that come with it,” Adams said. 

“For many insects that eat it, the toxin will block their sodium-potassium pumps. But this beetle developed a way to not only resist the toxin, but also sequester it, hold on to it, to keep the beetles themselves safe from would-be predators.”

Specialized diet of milkweed beetles

The experts also discovered variations in genes responsible for the beetle’s sense of smell, taste, and the metabolic enzymes needed to break down plant cell walls. 

This new understanding provides a fresh perspective for studying how beetles, particularly long-horned beetles and other plant-eating insects, have specialized their diets over time.

The findings could have significant applications in agriculture and forestry, particularly for managing pests. 

By identifying the genetic factors that allow agricultural and forestry pests to successfully feed on plants and evade control measures, researchers can develop better pest management strategies.

Valuable new insights

Most animals capable of digesting woody plant material rely on gut microbes to help break down plant cell walls. However, many plant-eating beetles do not need these microbes. 

Instead, they have evolved their own mechanisms, often acquiring the ability to break down plant material through horizontal gene transfers from microbes.

Adams pointed out that examining the diversity of proteins encoded within beetle genomes can provide valuable insights into the genomic basis of beetle biology, evolution, and interactions with plants. 

“Nature has made an incredible diversity of genes and genomes already out there that we have not yet deciphered,” said Adams. 

“Understanding this diversity holds great promise for informing agriculture, forestry, and human health. Herbivorous beetles would have a difficult time feeding on plants without their metabolic enzymes, because they can’t eat effectively without them.”

How milkweed beetles sense their environment

In addition to studying the beetle’s genomic DNA, the team collected RNA from both male and female beetle antennae to explore how the beetles use chemosensation – the ability to detect chemical signals in their environment – to find mates and food.

“Learning more about chemosensory biology – how an organism senses its environment, like sensing a host plant or reproductive partner – has broad relevance for understanding insect-plant interactions, which is intensively relevant to agriculture and forestry,” Adams explained.

The team’s RNA profiling provided the first transcriptomic resource for Tetraopes, offering new insights into the genes that are expressed in specific tissues or cells. 

As Adams noted, while DNA provides the gene sequence, RNA allows researchers to gain a clearer picture of how genes are expressed and how often a particular gene is activated.

This research opens new doors to understanding specialized plant-feeding in beetles and could eventually help in developing pest control strategies to protect crops and forests from invasive species.

The study is published in the Journal of Heredity.

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